Treatment of wells



Patented Feb. 25, 1941 irs stares war TREATREENT F WELLS John G. Staudt,Tulsa @kla'., assignor to The Dow Chemical Company, Midland, Mich acorporation of'Michigan No Drawing.

Claims.

The invention relates to an improved method of acid treating a welldrilled into or through a calcareous formation to increase the yield offluid mineral products therefrom. It more par- 5 ticularly concerns animproved method of acidizing oil and gas wells.

In the treatment of a well with an acid solution which is capable ofdissolving constituents of the productive earth and rock formationpenetrated by the well bore, thereby increasing the permeability of saidformation, I have found that in some instances the acid will drain intothe formation with relatively great facility and when this occurs anincrease in production is not often obtained. I have observed that thisdraining action may occur even in wells which at the time of treatmentproduce at a very substantial rate. It is evident therefore, that insuch cases pro- "duction is being derived'from the relativelyimpermeable sections and that the acid when introduced into the wellsimply drains into the relatively permeable portions withoutbeneficially affecting the tighter portions, for otherwise the action ofthe acid in dissolving the rock and en- 35 larging the pores of theformation would bring about an increased rate of production.

It is,v therefore, an object of the invention to provide a method oftreating well formations with an acid, whereby the acid reagent may besubstantially prevented from draining into the rela- I have now foundthat the foregoing objects 40 can be attained and greater increases inoutput obtained by introducing into the well along with the acid one ormore successive small portions or v slugs of a substantiallynon-penetrating liquid mixture substantially unafiected by the acid and46 having a specific gravity about the same as that -of the acid. Theterm non-penetrating liquid mixture" used herein and in the appendedclaims means a liquid mixture comprising a dispersion or solution in aliquid of a material which pre 50 vents the liquid from penetratingporous forma tions such as those tra ners/ed by a well bore, but ofsumcient'fluidity to be introduced into the well and readily withdrawntherefrom. The .nonpenetrating liquid mixture introduced along with 68the acid is carried into the relatively permeable Application February17, 1938, Serial No. 190,975

- casing and tubing, the acid solution is first introduced into thewell, preferably-through the tubing, and maintained under a pressuresufficient 15 to force the acid into the formation. When it is observedthat the acid is rapidly draining away into the formation under littleor no applied pressure, a quantity of a non-penetrating. liquid mixtureis introduced into the well, prefer- 20 ably in slugs, along with theacid. The vylume of the individual slugs generally may be from 5 to 30gallons, although larger slugs may be required in some cases dependingupon the area of permeable formation to be sealed. The non-penetratingliquid mixture flowing down the tubing in slugs with the acid is carriedby it to the highly permeable portions of the formation where it becomesfiltered out forming a sealing deposit. The

sealing deposit substantially prevents further entrance of the acid intothese sections and thereafter the acid solution is thereby directed intothe relatively tight unsealed portions of the formation. The injectionof acid is continued until the desired quantity has been injected intothe formation, usually 500 to 5000 gallons or more being employed, whileslugs of non-penetrating liquid mixture are introduced as often as itappears that the acid is draining away into porous sections of theformation, whereby loss of acid due to drain- 40 age into the poroussections is prevented. Usually it is necessary to add from 3 to 5 slugsof the non-penetrating liquid mixture during an acid treatment, althoughin some cases one slug of the said mixture is all that is required toproduce the desired results. After the charge of the acid is introducedinto the well, fluid pressure such as oil or gas may be applied todisplace the acid from the tubing, if necessary. The well is then closedin until the reaction is substantially to complete, after which thepressure may be released and the spent acid removed by pumping it fromthe well. When the well is put into production, the flow of .fiuids intothe well readily removes the non-penetrating liquid mixture deposited onthe walls of the well bore, thus allowing unhindered flow fromsubstantiallyall portions of the producing formation.

Among the non-penetrating liquid mixtures that are useful for preventingthe acid from penetrating into the more porous sections are those thathave substantially the same specific gravity as that of the acid beingused, preferably immiscible with the acid, and not attacked by it. Amixture having suitable characteristics may be made for example byforming a relatively thick emulsion of oil and water. Such an emulsionmay be made by passing the oil and water together through a mixing heador homogenizer at a high rate of speed in conventional manner. Any ofthe well-known emulsifying agents may be incorporated in the mixture tofacilitate the forming of the emulsion. Any suitable consistency can. beobtained by varying the proportion of the ingredients. Another'type ofnonpenetrating liquid mixture having gel-like characteristics may bemade by mixing from 3-5 per cent of certain soaps such as aluminumstearate and aluminum palmitate with a liquid such as kerosene,gasoline, benzene, and preferably crude oil because of its cheapness,and then heating the mixture to a temperature of from 50 to 80 0.,

after cooling to normal temperatures it is'ready I for use. Stillanother type of non-penetrating fluid, consists of a solution ordispersion of an organic jellifying material in water. By an or:- g'anicjellifying material is meant an organic material capable of beingdispersed or dissolved in water to make a mobile liquid dispersion orsolution, which liquid exhibits jellifying characteristics after a time.Suitableorganic jellifying materials may be prepared from starches, suchas those of cassava or rice, by mixing them with water to ,form a thinpaste and boiling the mixture until the starch is hydrolyzed. The cookedstarch is dried at about 80 C. and the dried .material ground,preferably to .a. fine powder. The powder so produced may be mixed withwater in a concentration of from about 1 to 8 per cent by weight to givea dispersion or solution of an organic jellifying material. Tests oncores secured from typical formations rendered relatively porous by acidtreatment, have shown.

that such non-penetrating liquid mixtures are not forced through thecores even when relatively high pressures are applied to the mixture.

Suitable acids to employ incarrying out an.

acid treatment according to the invention are those acids capable ofattacking and dissolving the calcareous rock, and preferably thoseforming water soluble salts therewlth, such as a 15% solution ofhydrochloric, acid. Other acids, such as nitric acid, or mixtures ofnitric and hydrochloric acids may be used.

Among the advantages of the invention are that relatively impermeablesections of the formation ordinarily not attacked by the acid are pene--plained, change .being made as regards the methodherein disclosed,provided the step or 1 steps stated by any of the following claims orthe equivalent of such stated stepor steps be employed.

I therefore particularly point out and distinctly claim as my invention;

' l 1. In a method of acid treating a well in a calcareous formation toprevent the loss of acid reagent by drainage into porous sections, thesteps which include introducing a quantity of acid into the well,thereafter continuing the introduction of acid while including therewitha relatively small quantity of substantially non-penetrating liquidmixture having about the same specific gravity as the acid, whereby thenonpenetrating mixture is deposited on the porous sections of theformation, sealing them against the flow of acid thereinto, andrepeating the foregoing steps as desired.

2. In a method of acid treating a well in -a calcareous formation toprevent the loss of acid reagent by drainage into sections beingrendered porous by the acid, the well being equipped with the usualcasing and tubing, the steps which include introducing a quantity ofacid into the well through the tubing, and while said acid is flowinginto the well including in the acid stream at least one slug of anaqueous dispersion of an organic jellifying material having about thesame specific gravity as that of said acid, whereby the organicjellifying'material is deposited on the porous sections of theformation, temporarily sealing them against the flow of acid thereinto.

3.-In a method of acid treating a well in a calcareous formation havinga section of relatively low permeability adjacent the bottom of the wellbore, to prevent the loss of acid reagent by drainage into, sectionsbeing rendered porous by the acid solution, the steps which includeintroducing a quantity of acid into the well and thereafter continuingthe introduction of acid while including therewith a small quantity of asubstantially non-penetrating liquid mixture having about the samespecific gravity as the acid,

4. In a method of acid treating a well in a calcareous formation toprevent the loss of acid reagent by drainage into the porous sections,

said well being equipped with the usual casing and tubing, the stepswhich include introducing a quantity of acid into the well through thetubing and while said acid is flowing into thewell including in the acidstream at. least one slug of a substantially non-penetrating emulsionhavingabout the same specific gravity as that of said acid, whereby theemulsion is carried into the well by the acid and deposited on theporous section of the formation sealing it against the flow of acidthereinto.

5. In a method of acid treating a well in a calcareous formation toprevent the loss of acid reagent by drainage 'into the porous sections,the

. well being equipped with the usual casing and tubing, the steps whichinclude introducing a quantity of acid into the well through the tubing,and while said acid is flowing into the well including in the acidstream at least one slug of non-penetrating mixture comprising oilandfrom 2 to 5 I per cent aluminum stearate, said mixture having about thesan e specific gravity as the acid, whereby saidmixture is deposited onthe porous sections of the formation sealing them against the flow ofacid thereinto.

JOHN G. STAUDT.

40- whereby the non-penetrating liquid mixture is

